[LoongArch] Allow writing unaligned nop sequences

In case of unaligned nop sequences, pad to the nearest 4-byte boundary
with zeros before filling with `nop` instructions. This is consistent
with gas behavior, and is necessary to compile the Linux kernel with
LLVM IAS.

Replace `support::endian::write` with `OS.write` while at it. This is
simpler and correct because we only have little endian.

Reviewed By: SixWeining, xen0n

Differential Revision: https://reviews.llvm.org/D139285